Power actuated valves of this general character afford many advantages, particularly in conjunction with remotely control operating systems. Characteristically, such valves are most reliable, nevertheless, operational failures can occur due to power failures or an occasional malfunction of the power driving mechanism or associated controls.
When normal power operation of such a power actuated valve is impossible on account of a power failure or otherwise, it is desirable and even essential that the valve be operated either to start or stop a flow of fluid through the valve. As an emergency procedure, such valves have been operated manually either by turning the driving motor or by turning the output end of the power drive connected with the valve control stem. Neither of these options has been satisfactory and have, on occasion, been ineffective and even a cause of damage to the mechanism.
On account of the wide difference between the rather limited output torque of the driving motor which is practical to use in such valves and the comparatively large torque required to turn the flow control member in the valve, particularly in the case of ball valves, it is highly advantageous to connect the driving motor to the flow control member by means of a transmission having a very high torque multiplying capability and incidentally correspondingly high speed reducing capabilities.
Consequently, the option of manually turning the driving motor or input end of the torque multiplying and speed-reducing transmission is necessarily a slow process because of the many turns required. Moreover, the torque manually applied to the driving motor end of the transmission and being possibly much higher than the maximum torque output of the driving motor is multiplied by the speed-reducing transmission with the consequence that the valve or its operating mechanism can be rather easily damaged by such manual operation.
On the other hand, manually turning the output end of the driving mechanism connected to the valve control shaft can be difficult and ineffective or even impossible as a practical matter. For example, the torsional drag of electrical brushes associated with an electric driving motor can, when multiplied through the speed-reducing transmission, impose a strong negative torque or braking effect resistant to turning of the output end of the drive connected to the valve control stem. As a practical matter, the cumulative drag of this negative torque of the unenergized drive mechanism and the normal turning resistance of the flow control member within the valve can make it unfeasible to manually open or close the valve by torque manually applied to the output end of the actuating mechanism.